The subject matter of the present invention relates to drill stem testing methods and apparatus, and, in particular, to apparatus and techniques for killing wells after testing. The invention is particularly suitable for pressure controlled testing systems but is not limited to such systems. The invention also relates to production well testing techniques, and to the testing of open hole sections.
Drill stem testing systems are well known and reference should be made by way of example to the applicant's prior publications European Patent EP-A-63519, U.S. Pat. No. 4,718,494 and U.S. Pat. No. 4,915,168, the contents of which are incorporated into this specification by reference. These three publications describe the principles of drill stem testing as well as the principles of the major drill stem test string components and their operation.
A drill stem test tool string suspends from a pipe string in a wellbore, and a packer is set thereby isolating a rathole from an annulus. The term "rathole" is defined to be the annular space which exists between the pipe string and a wall of the wellbore below the set packer in the wellbore. The term "annulus" is defined to be the annular space which exists between the pipe string and the wall of the wellbore above the set packer in the wellbore. Formation fluids (fluid received from an earth formation during testing) accumulate both in the rathole and in the pipe string. For safety reasons, it is necessary to remove these formation fluids from the rathole and pipe string in order to kill the well. Fluids are removed from the pipe string by reverse circulation of mud through one or more reversing tools which form a part of the tool string. A typical reversing tool is described in EP-A-63519 referred to previously. Initially, the tester valve above the set packer is closed, separating the rathole from the cushion. Conditioned mud is then pumped down an annulus area between the test string and the well casing, i.e. through the reversing tool and into the test string thereby forcing formation fluids out through the top of the string. Reverse circulation continues until all formation fluids have been removed. Since the mud is not necessarily homogenous, some filtration is probable and it is common practice to pump at least 1.5 times the tubing volume during reverse circulation to ensure complete removal of formation fluids.
In order to restore the mud in the annulus and the tubing to their original conditions, mud is then circulated down the drill pipe tubing through the reversing valve and up the annulus. Finally, the rathole must be equalized. Under usual drilling conditions, the formation zone is relatively small and equalization is achieved by forcing formation fluids back into the surrounding formation. To do this, the seals between the packer and the well casing are released and mud is pumped from the annulus into the rathole between the packer and the casing. As the hydrostatic pressure of the annulus is much greater than the formation pressure in the rathole, this operation is safe and ensures removal of formation fluids.
U.S. Pat. No. 4,718 494 referred to previously describes a type of tool which is controlled by annulus pressure. This tool is one of a class of tools which together make up a drill stem test (DST) string. This tool reduces the need for string movement and is particularly suited to use on offshore floating rigs. Some of the tools are operated by overpressurization of the annulus, for example, to burst a rupture disk in a valve.
A variant of the DST string is the tapered test string. This string is suitable where very narrow bores must be drilled, for example, to overcome geological difficulties preventing the usual 7" or 95/8" casings from being used. In such areas, a casing is sunk which has an external diameter of 5 inches or smaller. However, the external diameter of standard DST tools is 5.0 inch and they cannot therefore be used in these small bore casings. As the internal diameter of 7 inch casing is 5.89 inches, the clearance is small even under usual conditions.
To overcome this problem small bore DST tools have been developed which have an external diameter of 33/8 inch and an internal diameter of 1/2 inch. However, the size restrictions on these tools are such that they are not as satisfactory as the 5 inch standard tool. It is therefore sometimes preferred to operate a tapered test string which comprises a string of standard 5.00 inch tools above a fixed packer higher up the hole in the larger diameter 7 inch casing and a string of narrow gauge tubing in the rathole.
Although it is possible to set a packer in a 5 inch casing or smaller, it is preferable to locate the packer in the wider bore section of the well. This means that the rathole beneath the packer is the complete length of the narrow bore section. The production packer has a smooth inner surface which allows an assembly to locate and seal inside it.
In some cases, the 5 inch casing may be up to 2000 ft. in length. Under these circumstances, the technique described previously for killing the rathole is no longer practical as there may no longer be a sufficient pressure difference across the packer to ensure that the hydrostatic pressure in the annulus will retain the formation fluids in position on release of the packer seal. The consequences of releasing the packer seal under these conditions could be catastrophic, resulting in a blow out. A further problem arises in that the formation around the rathole can act as a one way valve, resisting attempts to force large amounts of formation fluids back into the formation rock.
In view of these problems, it is not safe or desirable to release the packer seal to pump in annulus mud, the technique which is usually used in the short rathole example given above. Attempts have been made to overcome the problem using a hold open (HOOP) in a pressure controlled tester (PCT) downhole tester valve. This allows annulus pressure to be bled while keeping the valve fully open. To reclose the PCT, all that is required is the repressurization of the annulus and further bleeding. The normal open/close sequence can be continued until the hold open cycle is reached again. As a result, the industry has identified a need for a reliable, safe method and apparatus for removing formation fluids from a long small diameter casing.